Abstract
Hydroxyapatite (HAp), Ca10(PO4)6(OH)2, is known to precipitate on bioactive materials such as TiO2 and CaTiO3 by soaking in simulated body fluid (SBF). The formation of HAp on TiO2 surfaces under continuous ultraviolet (UV) irradiation was investigated. Anatase-type TiO2 film was synthesized on pure Ti substrates by a combined chemical-hydrothermal treatment. The specimens were immersed in SBF in darkness or under UV irradiation with a centered wavelength of λ = 365 nm. Under dark conditions, a thin homogeneous HAp film was formed, with just a few spherical clusters of HAp. The UV irradiation promoted the formation of HAp clusters, which may be due to the generation of functional Ti-OH or Ti-OH- groups on the TiO2 surface. The UV light produces electron-hole pairs in the TiO2. When an n-type semiconductor is immersed in an aqueous solution, an up-hill potential gradient is produced towards the surface in the conduction and the valence bands. Therefore, the photogenerated holes migrate to the surface and repel the Ca2+ ions in the solution near the surface of TiO2. As a consequence, the UV irradiation suppressed the formation of a HAp thin film.
